Devices and processes are known in which glass sheets are carried through heating furnaces by conveyor rollers and in some instances gas cushions are used to move glass sheets through at least the hottest part of the furnace. Individual regulation of the temperature of each of the glass sheets has not been achieved in these furnaces. It is essential, however, to bring the glass sheets to as exact a temperature as possible in order to obtain a uniform degree of tempering. Too low a temperature causes insufficient tempering and too high a temperature runs the risk of deformation of the sheet particularly when it is carried flat on rollers. Actually, to obtain a glass sheet of good optical quality and which at the same time has the desired shattering properties, it is necessary to stay within a very narrow temperature bracket which does not exceed a few degrees.
In a tunnel type furnace, sufficient temperature constancy can be attained only if, on the one hand, the furnace is constantly supplied with energy and, on the other hand, the speed of movement of the glass sheets through the furnace is constant so that an energy balance is established. This also means that, to maintain this balance, it is necessary to treat only sheets of the same size and immediately modify settings to establish a new balance if the size is changed. However, the energy balance in the furnace can also be destroyed if the spacing between the glass sheets varies.
It has been found very difficult to arrange sheets manually with constant spacing or by known automatic devices. When it is considered that to feed modern tunnel-type furnaces, six hundred sheets or more per hour must be placed on a conveyor at a uniform rate, it can be seen that only a short period of time is available to place and position each of the sheets which in large measure explains the difficulties.
It is an object of the invention to improve thermal balance of a bending and/or tempering furnace to attain a better temperature constancy of the sheets arriving at bending and tempering stations and thereby achieve a uniform quality of tempered glass sheets from the viewpoint both of optical properties and degree of tempering.